The lipid composition of gallbladder bile, bile acid pool size and sex hormone levels will be determined in Pima Indians below 21 years of age to determine the age at which Pima Indian bile becomes lithogenic. These data will indicate the approximate duration of bile lithogenicity prior to the appearance of cholesterol gallstones and indicate the period of life at which prophylactic measures against gallstone formation may be taken. Bile will be obtained by duodenal siphonage after stimulation of gallbladder contraction and bile acid pool size will be measured to determine the ratio of deuterated chenodeoxycholic acid to native chenodeoxycholic acid in bile twelve hours after the intragastric administration of the deuterated form. The observed changes will be correlated with age, level of sex hormones, pubertal history, body size and sex.